1. Field of the Invention
Embodiments of the present invention relate generally to variable compression ratio (VCR) internal combustion engines and, specifically to VCR internal combustion engines with movable crankshafts for varying the compression ratio.
2. Background of Related Art
In a reciprocating internal combustion engine, the compression ratio of an engine is defined as the ratio between the free volume of the cylinder when the piston is at bottom-dead-center (BDC) and the free volume when the piston is at top-dead-center (TDC). All other things being equal, engines tend to be more efficient and produce more power when run at higher compression ratios because this results in higher thermal efficiency. Diesel engines, for example, run at very high compression ratios (18:1 and higher) resulting in compression ignition (i.e., spark plugs or other ignition sources are not required to light the fuel). The higher compression ratio of diesel engines, along with the slightly higher heat content of diesel fuel, results in an engine that provides significantly better fuel mileage than a comparable gasoline engine (30% or more).
In a gasoline engine, however, increasing the compression ratio is limited by pre-ignition and/or “knocking.” In other words, if the compression ratio is high enough then, like a diesel, the compression of the fuel causes it to ignite (or, “pre-ignite) before the spark plug fires. This can result in damage to the engine because cylinder temperatures and pressures spike as the fuel/air mixture explodes on multiple fronts, rather than burning uniformly. The maximum acceptable compression ratio in an engine is limited by a number of factors including, but not limited to, combustion chamber and piston design, cylinder and piston cooling, engine loading, and air temperature and humidity. The maximum compression ratio used in production engines is generally relatively conservative (on the order of 10.5:1 for cars and 12.5:1 for motorcycles) to account for, for example, the wide variety of operating conditions and fuel quality.
Due to difficulties associated with reliably moving components in an operating internal combustion engine, however, all currently mass produced engines operate with a fixed compression ratio. As a result, the stock compression ratio tends to be a compromise between a high-compression ratio, which is more efficient—but can result in the aforementioned knocking—and a low compression ratio engine—which is more forgiving of, for example, poor quality fuels, high loads, and/or high temperatures—but has lower efficiency.
The ability to change compression ratio during operation can improve fuel efficiency 35-40% and more. When under light load, such as when the vehicle is cruising down the highway, for example, the compression ratio can be increased significantly to increase fuel mileage. When the engine is under a heavy load, ambient air temperature is very high, or fuel quality is low, on the other hand, the compression ratio can be reduced to prevent knocking. The ability to change compression ratio during operation also allows turbocharging, supercharging, and other power adders to be incorporated much more efficiently.
A number of designs exist that have attempted to vary the compression ratio of an internal combustion engine. Patents have been filed on variable compression ratio engines (VCRE) for over 110 years. A few of the proposed VCRE engines are based on the concept of raising and lowering the cylinder block/head assembly portion of an engine relative to the crankcase. In this configuration, the distance between the piston at top-dead-center (TDC) and the cylinder head can be varied, thus varying the compression ratio of the engine.
Several designs, such as U.S. Pat. No. 6,990,933 B2 and General Motors' DE 10 2009 038 180 A1, filed Mar. 24, 2011 entitled, “Fahrzeugmotor mit einem Kurbeltrieb für eine variable Verdichtung” (“Vehicle Engine with a Crank Mechanism for a Variable Compression”) (the '180 Patent), achieve variable compression by moving the crankshaft vertically (or substantially vertically) with respect to the cylinder head. This configuration, however, presents challenges with respect to taking power off the engine. In other words, conventional, fixed ratio engines connect to a clutch or torque converter on the rear of the engine and an accessory drive (i.e., for driving alternators, power steering pumps, etc.) on the from the of the engine. In order to effectively couple and seal the cranks shaft at either end, however, it is generally necessary for the crankshaft to rotate about a stationary axis. The '180 Patent proposes a gear driven slave shaft to account for this motion. This requires turning the power from the crankshaft through 180 degrees (90 degrees per gear) and adds inertia and complexity to the system.
What is needed, therefore, is a system for varying the compression ratio of an internal combustion engine without unnecessarily increasing the weight or complexity of the engine. The system should enable a portion of the crankshaft containing the crankpins (e.g., the portion proximate the connecting rods) to move vertically with respect to the cylinder head, yet continue to provide the portions of the crankshaft rotating about a fixed axis. In this manner, the ends of the crankshaft can extend through the engine block and be sealed in the conventional manner. Essentially, what is needed is an engine that creates variable compression ratio, yet provides at least a portion of the crankshaft to be rotating about a fixed axis in a fixed location in the block as in conventional engines for the past 125 years. The system should use conventional manufacturing techniques to provide easily manufacturable, reliable engines with, among other things, improved power-to-weight ratios, and fuel consumption. It is to such a system that embodiments of the present invention are primarily directed.